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Table Of Contents  The TCP/IP Guide
 9  TCP/IP Lower-Layer (Interface, Internet and Transport) Protocols (OSI Layers 2, 3 and 4)
      9  TCP/IP Network Interface / Internet "Layer Connection" Protocols
           9  Address Resolution and the TCP/IP Address Resolution Protocol (ARP)

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TCP/IP Address Resolution For IP Multicast Addresses
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Reverse Address Resolution and the TCP/IP Reverse Address Resolution Protocol (RARP)
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TCP/IP Address Resolution For IP Version 6
(Page 2 of 2)

Using Solicited-Node Multicast Addresses For Resolution

The solicited-node multicast address is a special mapping that each device on a multicast-capable network creates from its unicast address; it is described in the topic on IPv6 multicast addresses. The solicited-node address isn't unique for every IPv6 address, but the odds of any two neighbors on a given network having the same one are small. Each device that receives a multicasted Neighbor Solicitation must still check to make sure it is the device whose address the source is trying to resolve. (This is similar to how multicast is handled in IPv4, with 32 different IP addresses potentially sharing a multicast MAC address.)

Why bother with this, if devices still have to check each message? Simple: the multicast will affect at most a small number of devices. With a broadcast, each and every device on the local network would receive the message, while the use of the solicited-node address means at most a couple of devices will need to process it. Other devices don't even have to bother checking the Neighbor Solicitation message at all.

Key Concept: Address resolution in IPv6 uses the new Neighbor Discovery (ND) protocol instead of the Address Resolution Protocol. A device trying to send an IPv6 datagram sends a Neighbor Solicitation message to get the address of another device, which responds with a Neighbor Advertisement. When possible, the request is sent using a special type of multicast address rather than broadcast, to improve efficiency.


This is actually a fairly simplified explanation of how resolution works in IPv6—the Neighbor Discovery protocol is quite complicated. Neighbor solicitations and advertisements are also used for other functions such as testing reachability of nodes and determining if duplicate addresses are in use. There are many special cases and issues that ND addresses to ensure that no problems result during address resolution. ND also supports proxied address resolution.

Note: Even though I put this topic where it would be near the other discussions of address resolution, the Neighbor Discovery protocol really isn't a “layer connection” or “lower level” protocol like ARP. It is analogous to ICMP in its role and function, and in fact makes use of ICMP(v6) messages. One advantage of this architectural change is that there is less dependence on the characteristics of the physical network, so resolution is accomplished in a way more similar to other network support activities. Thus it is possible to make use of facilities that can be applied to all IP datagram transmissions, such as IP security features. The section on ND contains much more information on this subject.


 


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Reverse Address Resolution and the TCP/IP Reverse Address Resolution Protocol (RARP)
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